docs: def add_analysis_subgroup(self, group_name, subgroup_name, attrs=None):
          """ Add a new subgroup to an existing analysis group.
          
          :param group_name: The name of the analysis group you are adding to.
          :param subgroup_name: The name of the new subgroup.
          :param attrs: A dictionary representing the attributes to assign to
              the subgroup.
          
          The new subgroup must not already exist.
          
          The subgroup name can be a nested name, such as "Template/Data". This
          will create the "Template" subgroup (if it does not exist), and the
          "Data" subgroup below it.
          """
          self.assert_writeable()
          group = 'Analyses/{}/{}'.format(group_name, subgroup_name)
          self._add_group(group, attrs)